Molten metal that is discharged from a melting furnace has high cleanliness since inclusions such as hydrogen gas, oxides and intermetallic compounds are removed by a flux treatment and degassing. However, in low-pressure casting methods, the cleanliness of the molten metal is gradually decreased, since contact between molten metal and air is inevitable.
A core, which is disposed inside a mold, contains water, resin and the like. Such water, resin and the like are vaporized by the heat of molten metal to produce gas. When such gas is left inside a molded product, it causes a gas defect or a shrinkage cavity so that the quality of the molded product is decreased.
In particular, since water also produces hydrogen gas that causes hydrogen embrittlement of a molded product, it is important to remove water and the like that are vaporized by heat of molten metal in order to improve the quality of the molded product.
However, water is also included in the air, and some air is taken into a cavity when the mold is opened. Further, in order that a core to be disposed inside a mold does not contain water, it is necessary to store the core in a humidity-controlled room, which requires a large cost for the storage of cores.
Although it is not related to low-pressure casting, JP H08-33944A discloses installing a pipe for suctioning gas inside the bodies of a mold and a core made of casting sand, and vacuuming the bodies of the mold or the core to partly reduce the pressure while supplying molten metal to a cavity, so as to suction gas that is generated from the bodies of the mold and the core. It discloses that this method can prevent gas that is produced by thermal decomposition of an organic binder contained in the mold and the like, from penetrating into molten steel, and thereby can prevent gas defects.
Although it is not related to low-pressure casting either, JP 2014-136245A discloses using an adsorbent such as zeolite or ALC instead of a technique of feeding hot air to a cavity to dry a sand mold that defines the cavity, since such techniques can remove only water that is present in the surface layer of the sand mold.
That is, it discloses surrounding the casting sand of the mold by the adsorbent such as zeolite or ALC and allowing the adsorbent to adsorb and remove water of the casting sand even to the inner part. Further, it also describes forming a core, if employed, from a sand mold of casting sand, an adsorbent embedded inside the sand mold and a reinforcing steel embedded inside the adsorbent.
However, in the method of JP H08-33944A, the molten metal may be suctioned into gaps between the casting sand to cause a sand mark, or insufficient pressure reduction may cause a gas defect.
That is, it is difficult to uniformly reduce the pressure in the bodies of the mold and the core, and the inner pressure of the mold and the like tend to vary. Further, gas from the mold and the like is not only derived from the organic binder but also produced from water contained in the mold and the like. Since the water content changes depending on the storage environment of the mold and the like, it is also difficult to estimate the amount of gas to be produced by pouring molten metal beforehand.
In the method of JP 2014-136245A, the adsorbent has a certain capacity with regard to the amount of adsorption, and it is necessary to store the mold and core so that the mold and core do not absorb water over the capacity of adsorption of the mold and core. Furthermore, it requires considerable manpower to produce the mold and core, which increases the cost.
The present invention has been made in view of these problems with the prior art. It is an object of the invention to provide a low-pressure casting method and a low-pressure casting apparatus that do not require any special processing, such as piping, other than shaping of a mold and a core, while it can reduce gas produced by heat of molten metal so as to prevent gas defects and shrinkage cavities, and it facilitates storage of the core.
As a result of a keen study for achieving the above-described object, the present inventors have found that the above-described object can be achieved in a low-pressure casting method by reducing the pressure in a cavity to dry a core after disposing the core in a mold and closing the mold and before filling the mold with molten metal. The present invention has been thus completed.